Replace Object.assign in common, since it's not supported in IE

[celebro]

Object.assign() is not available in IE11. In environments without polyfills this library crashes on IE. This PR replaces it with a for-in loop.

[coveralls]

Coverage decreased (-0.2%) to 96.073% when pulling 67e19d8 on celebro:fix/remove-object-assign-for-IE into 6e60eb5 on mweststrate:master.

[scriby]

Can you post benchmarks before and after this change?

[mweststrate]

Merged, thanks @celebro!

@scriby tested, doesn't seem have real impact, but might a small change that it doesn't try to polyfill it by default